Hydrogen rich, hence the need for the sparklers
I think alot of folks still think the sparklers ignite the engines, but they actually ignite any free, unburned hydrogen under the engine nozzles so there's not an explosion down there at engine start. Just wanted to clarify that for anyone 'new' to shuttle launches.
So what does ignite the engines?
Big spark plugs/torch ignitors that fire in the pre-burner inside the SSME.
From the Space Shuttle Main Engine Orientation Presentation (105 pages) on L2:
"ASI SPARK IGNITER
The spark igniter generates sparks at a rate and energy level sufficient to ignite the propellants in the
Six igniters are used on the SSME, two each (for redundancy) in the MCC and both preburners. The
spark igniter is completely self-contained in that a 26-VDC input as provided by the controller, results in
a 10-kilovolt, 50 sparks per second output. The igniter is hermetically sealed and welded."